Active matrix-type liquid crystal display devices such as LCD uses thin film transistors: Thin film transistor (hereinafter also referred to as TFTs.) as switching elements. A schematic cross-sectional view of the TFT element is shown in FIG. 1. The TFT element comprises a gate electrode 6 formed on a glass substrate 5, a non-doped semiconductor layer 1a provided on the electrode 6 via a gate insulating film 7, and an impurity-doped semiconductor silicon layer 1b which is in contact with the non-doped semiconductor silicon layer 1a. The non-doped semiconductor silicon layer 1a and the impurity-doped semiconductor silicon layer 1b are collectively referred to as a semiconductor layer 1. This impurity-doped semiconductor silicon layer 1b is electrically connected by a metal wire of Al alloy or others. These metal wires are referred to as a source electrode 2 and a drain electrode 3. A transparent conductive film 4 used in the liquid crystal display unit is further connected to the drain electrode 3.
Various Al alloys have been heretofore suggested as materials used for these source electrode and drain electrode, as in patent documents 1 and 2, among others. The source electrodes and drain electrodes described in these prior art documents have a multilayer film comprising high-melting-point metal such as Mo, Cr, Ti and W interposed therebetween as a barrier metal so that they do not come in direct contact with the transparent conductive film (hereinafter also referred to as ITO film.) used for semiconductor layers and liquid crystal display units.
Various techniques of eliminating a barrier metal existing between the drain electrode and transparent conductive film (ITO film) have been suggested, for example, in patent documents 3, 4 and 5, among others. However, techniques for eliminating the barrier metal provided between the source electrode or drain electrode and the semiconductor layer of the thin film transistor have not been discussed sufficiently. Moreover, for active matrix type liquid crystal display devices, requirement for refinement mainly in mobile applications is high, and micromachining (in particular dry etching) of metal wires such as source electrodes and drain electrodes are highly required. However, this aspect has not yet been discussed sufficiently.
[Patent document 1] Japanese Unexamined Patent Publication No. 1995-45555
[Patent document 2] Japanese Unexamined Patent Publication No. 2005-171378
[Patent document 3] Japanese Unexamined Patent Publication No. 2004-214606
[Patent document 4] Japanese Unexamined Patent Publication No. 2005-303003
[Patent document 5] Japanese Unexamined Patent Publication No. 2006-23388